The invention concerns optical lines for communication and for sensing in which a communication or sensing light signal is transmitted by an optical fiber or by a plurality of optical fibers connected together. So-called special optical fibers having a specific function are often inserted into the length of a communication or sensing line made up of conventional optical fibers (called normal fibers because their main function is to transmit the light signals), or else they are connected to one end of the line.
Special optical fibers can have several functions:
they can act as sensors, especially if the optical line in question is a sensing line, PA1 they can act as optical amplifiers, optical isolators, optical circulators, etc inserted into the length of a communication line, PA1 in the case of interconnecting two communication lines having different core diameters, they can be used to match the core diameters to each other, PA1 they can be used to increase the light power injected into a communication or sensing line. PA1 use is made of a vertical cylindrical cavity lined with porous layers whose inside dimensions are a few tens of microns larger than said part and in which can slide a cylindrical pedestal constituting a support for said part, PA1 a seed mass of said glass is provided on said pedestal, PA1 said seed mass is heated until it melts and a gas is injected continuously into said porous layers to maintain between them and said molten seed mass a layer of gas a few tens of microns thick preventing any contact between them, PA1 said molten seed mass is fed from the top of said cavity by continuously dispensing a powder made up of the constituents of said glass, and the composition of the powder is varied as said cylindrical part is formed, so that said glass constituting said cylindrical part has a composition that varies in the longitudinal direction, PA1 said pedestal is lowered as said cylindrical part is formed from said seed mass. PA1 the pedestal is a hollow cylindrical body closed at the bottom by a piston, PA1 the pedestal initially contains a solid charge of glass adapted to form said seed mass, PA1 the seed mass is extracted gradually from the pedestal in the molten state by pushing up the piston.
U.S. Pat. No. 3,966,446 describes a method of manufacturing an optical fiber preform whose refractive index varies longitudinally, a fiber of this kind being used, for example, to reduce mode dispersion in transmitted optical signals. The method described entails depositing glass soot whose composition is varied in time, for example by adding dopants to modify the refractive index in the axial direction of the preform.
That prior art method cannot produce fibers of the required quality if the parent glass is a multi-component glass.